This invention concerns a hydrocarbon conversion process assisted by special gliding arc plasma in the presence of carbon dioxide (CO2) and/or water vapor. This process is illustrated by the conversion of two model mixtures in an arc reactor equipped with a maturation post-plasma compartment:
a natural gas containing mainly methane and some ethane, propane and butanes, PA1 a "propane" containing some ethane and butanes. Therefore, the invention can be applied to any pure hydrocarbon, such as CH4, C2H6, C3H8 or C4H10 and to their mixtures. PA1 For the CH4/CO2 systems, mostly H2, CO, C2H2, with presence of C2H4 (&lt;5%) and of C2H6 (&lt;1%). PA1 For the CH4/H2O system, the same as above but with a few traces of CO2.
In the presence of water vapor and/or of CO2, it is then possible to convert, totally or partially, all these hydrocarbons basically into synthesis gas (consisting of a majority of hydrogen H2 and of carbon monoxide CO), but also into other valuable products, such as ethylene (C2H4), acetylene (C2H2) and propane (C3H6), and all without using traditional catalysts. The process is based mainly on steam reforming reactions, such as: EQU CH4+H2OVAP=CO+3H2 (1) EQU C2H6+2H2OVAP=2CO+5H2 (2) EQU C3H8+3H2OVAP=3CO+7H2 (3) EQU C4H10+4H2OVAP=4CO+9H2 (4)
reforming reactions with carbon dioxide, such as: EQU CH4+CO2=2CO+2H2 (5) EQU C2H6+2CO2=4CO+3H2 (6) EQU C3H8+3CO2=6CO+4H2 (7) EQU C4H10+CO2=8CO+51H2 (8)
cracking reactions, such as: EQU 2CH4=C2H4+2H2 (9) EQU 2CH4=C2H2+3H2 (10) EQU C2H6=C2H4+H2 (11) EQU C2H6=C2H2+2H2 (12) EQU C3H8=C3H6+H2 (13) EQU C4H10=2C2H4+H2 (14) EQU C4H10=2C2H2+3H2 (15)
as well as single and inverse water shift: EQU CO+H2O=CO2+H2 (16) EQU CO2+H2=CO+H2O (17)
All these reactions are performed in a medium highly activated by the presence of a special plasma produced by the gliding electric arcs. The activation of the medium is evident by the presence of rather unusual species (with respect to the traditional hydrocarbon conversion conditions) originating from the matter in which these arcs are developed. Thus, electrons can be detected, as well as atoms, ions and/or molecular radicals such as: H, OH, O, O2, H+, O+, O2+, OH-, HO2, CH3, CH2, CH, C2 and many others. Most of these species can exist in their excited electronic or vibrational states with a very long lifetime. They are also known as being extremely active chemically.
The production of synthesis gas starting from light saturated hydrocarbons is a very well-known and very important stage, especially for the upgrading of natural gases. The most used process at the present time, the catalytic steam reforming (or "steam reforming) encounters major problems. In principle, a high temperature (thermodynamic ratio) and a high pressure (for kinetic ratios) are sufficient for this process. However, in practice, despite the know-how for the production of "synthesis gas" according to the processes, the joint management of the compositions, pressures and temperatures is delicate, even impossible without resorting to catalysts.
Then, in order to perform natural gas (mainly rich in methane) reforming with water vapor, usually a catalytic way is sought: presence of an active solid substance for temperatures which can be attained without too much difficulty. Therefore, the traditional steam reforming technology uses furnaces in which several hundred fragile metal tubes (filled with a catalyst and having a length which can reach several dozen meters) are located, heated with natural gas. This technology is tied to the very strong drops in pressure and, especially, in heating energy. The temperature which the furnace pipes can withstand prevents also the reduction of CO2 content (awkward product originating from a parasite reaction at too low a temperature).
Other problems are connected with catalyst poisoning (by sulfur and/or nitrogen), with catalyst aging, with the necessary excess of water vapor and/or with the formation of soot which blocks the tubular system at a macroscopic scale and, most of all, the microscopic pores of the catalyst. These problems are observed particularly with steam reforming of hydrocarbons heavier than methane; they are more fragile and, hence, more coking.
The conversion of hydrocarbons according to the endothermic reactions (1) through (15) requires a supply of energy (preferably "clean"), without connection with any internal or external combustion. The best way to promote these reactions would be to strike electric arcs directly in the medium to be converted, imposing a permanent distribution of energy in the largest volume to be treated. The transfer of energy of electric origin to the gas mixture would be made by direct transfer of the energy to the molecules. This would result in excitation, ionization and dissociation phenomena and also in part by Joule effect, considering the ionized mixture as a gaseous conductor. This is to say that the gaseous mixture, which has been made into a conductor after ionization, itself due to dielectric breakdown (hence, a preionization) between electrodes brought to different potentials, would be considered as an electric resistance and, at the same time, as a sort of electrolyte in gaseous phase: the plasma.
Plasma is defined as the fourth state of matter and, therefore, cannot in any case be taken as a criterion of similitude for previously known different processes. Wanting to claim the concept of plasma or any type of reaction capable of developing at the plasma state, comes to wanting to claim all the reactions developing at the liquid state . . . There are one thousand one types of plasma, and one thousand one ways to obtain these plasmas. By definition (simplified), plasma is a gaseous medium in which the particles are in part ionized. Likewise, a part of the electrons is not associated with an atom, a molecule, an ion or a radical. Thus, even though globally, at a scale of a few microns, the medium is electrically neutral, two large families can be defined, in a simplistic way: the heavy particles (radicals, atoms, molecules and ions) and the electron cloud.
In most plasmas, the main macroscopic physical parameter--temperature--is the same for all the components: this is thermodynamic equilibrium. These conditions can be very easily obtained: it is sufficient to supply much energy, as in the case of plasma torches (plasmatrons, for some), where the plasma is produced by a very high electric arc current. There are also other devices capable of generating this state, such as, for example, induction or radiofrequency torches whereby the gaseous medium becomes resonant with an electric circuit. Such plasmas are called thermal plasmas by the experts. It is obvious that a thermal plasma will modify the chemistry of a gas medium, simply by destroying all the molecules, particularly the fragile ones, such as the hydrocarbons. The fragments found at the end of the process originate from partial recombination phenomena, often yielding too simple molecules. Such chemistry offers very poor prospects, requires much energy and presents problems connected with the high temperature (such as the resistance of the materials).
Professional chemists indisputably prefer the idea of a plasma which does not respect the conditions of complete thermodynamic equilibrium. For example, it is sufficient to act on the free electrons by taking advantage of the fact that they are much lighter. It is also possible to act on the rotation or vibration properties of some molecules. In terms of energy, this comes to breaking the energy exchange equilibrium between the plasma and the surrounding medium (heat, electrical energy, radiation, etc.). This state is qualified as non-equilibrium. Such plasmas are often called "low temperature" plasmas, although the concept of temperature cannot be used: there are several methods whereby such plasmas may be generated: microwaves, electron beams, flame front, etc. However, the generators of these plasmas are rare on an industrial scale and are appropriate only for a very precise application. This is the reason why, despite the great number of patents, such plasmas are rarely used in chemistry.
Also, when a plasma is established or when its existence is ended, the equilibrium is broken. These transitory states are actually non-equilibrium plasmas and last only a few milliseconds. One type of plasma takes advantage of this phenomenon, the gliding electric arc plasma, known under the name of "GlidArc", a relatively recent invention (1968) by H. LESUEUR et al. ["Low Temperature Plasma Generation Device through the Formation of Gliding Electric Discharges", BF 2,639,172]. Outside of the numerous geometric possibilities of a GlidArc plasma generator, and in a very global way, the parameters on which a chemist can act are: pressure, temperature, gas speed, current, electrical frequency and voltage. Such a number of parameters exceeds the conventional reasoning capabilities of the man of the trade. For each application, a real know-how and an inventive activity are necessary in order to obtain a result the objectives of which are both the economic profitability and the respect of the ecological principles. The approach allowed by the GlidArc enables the chemist to envision the distribution of a supply of energy directly in the gaseous mixture without, for example, resorting to catalysts. The chemist can also (to a certain extent) distribute directly the energy either in thermal form or in chemical form. He can also intervene on the flux still loaded with active species leaving the gliding arc zone, to have these species reach with the load to be converted in a maturation post-plasma zone.
Our bibliographic research concerning the last three decades yields few published and/or patented results concerning the partially oxidizing conversion of saturated hydrocarbons assisted by plasma. This may be due to the problems connected with the presence of oxygen originating from dissociation of the H2O and/or CO2 molecules and attacking the traditional tungsten or graphite electrodes of classic plasma devices. Nevertheless, we report these attempts to use different sources of plasma. Systematically, both the approach and the reaction process are different from ours. They have only point in common: the use of the word "plasma" or the possibility of treating the same hydrocarbon molecules.
K. KARL et al. [ . . . , CH 378,296 (1957)] proposed hydrocarbon steam reforming under 66.7 kPa-0.3 MPa pressure, in a "silent" discharge characterized by a 0.3-0.5 MV/m very intense electric field. This source of plasma has been known for a century and is totally different from that of the invention.
R. J. HEASON presented, in 1964, his doctorate thesis concerning methane pyrolysis and the reaction of CH4 with water vapor in an arc plasma (700 A, 20 V) in argon. These results are published only in manuscript form ["Investigation of methane and methane-steam reactions in an argon plasma", Dissertation, Ohio State Univ., Columbus]. A "thermal" plasma and a device consuming a great quantity of argon (2 moles Ar for 1 mole CH4) are involved.
C. H. LEIGH and E. A. DANCY ["Study of the reforming of natural gas by a plasma arc", Proc. of the Int. Round Table on Study and Appl. of Transport Phenomena in Thermal Plasmas, contribution 1.5, Odeillo, 1975, 11 pages] heated a mixture of CH4/CO2.about.1 in a jet of argon plasma, a traditional plasma arc torch. The jet temperature was approximately 10 kK. The argon flux was of the same order of magnitude as that of the mixture to be treated. These researchers observed a 11-74% conversion of carbon to H2, CO, C2H4 and C2H6 (without having ever detected C2H2 or H2O in the products?). No application was possible because of the high consumption of electrical energy (70% of it passed in the plasma torch cooling water) and of noble gas.
Also P. CAPEZZUTO et al. ["The oxidation of methane with carbon dioxide, water vapor and oxygen in radio-frequency discharges at moderate pressures", 3rd Int. Symp. of Plasma Chemistry, Limoges, 1976, contribution G.5.11, 7 pages] studied partial oxidation of methane placed separately in mixture with CO2, either with O2 or with H2O, with the ratios CH4/oxidizer=1. The 35 MHz radiofrequency (RF) plasma reactor needed an additional argon flux and could only work at low pressures of approximately 2.7 kPa. For a 3 to 36 l(n)/min total flow of entering gas, the energy density varied from 1 to 12 kWh/m3. No industrial use was possible because of the high consumption of electrical energy and of noble gas (in addition to the complexity of the electrical supply and the requirement to work under vacuum). The mechanical setup constraints, the low energy yield and the insufficient unit powers of the sources of RF plasma make the use of this method economically poorly suited for the transformation of major volumes of gas. However, it is interesting to note that, in all the cases, the authors observe an almost total conversion of the methane and an appearance of the following products:
A patent by S. SANTEN et al. ["Thermal reforming of gaseous hydrocarbon" GB-A-2 172,011] of 1986, claims the use of a plasma generator to heat reagents (a gaseous hydrocarbon, some water vapor and, possibly, some coal), completely or partially, up to a temperature exceeding 1200.degree. C. At such temperatures, these inventors expect favorable conditions to carry out their purely thermal process without the use of catalysts. The temperatures reached in the reactor and the thermal mode of the reforming (claimed and even emphasized in the title of the patent), therefore indicate a treatment of hydrocarbons under thermodynamic equilibrium. The process is based on a direct arc (two annular electrodes) or transferred arc plasma generator, which are very traditional devices known for almost a century.
L. KERKER writes in a general manner on the tests on production of synthesis gas at Huls [. . . in German . . . ]. The illustrations indicate that a tubular reactor with traditional arc, with very high power (1 to 9 MW), is involved; it has been used at this plant since 1939 to produce acetylene. This time, the case involved is a natural gas steam reforming project for the production of 99.9% pure hydrogen, at a very competitive price with respect to electrolysis (although still more expensive than the hydrogen generated by the traditional steam reforming or partial oxidation methods).
Our team in Orleans has also been working since 1986 on the conversion of hydrocarbons in thermal plasma reactors. These traditional torches with simple or transferred arc plasma make it possible to obtain plasmas with relatively small volume, but at very high temperatures (T&gt;10 kK). Although these devices may be potential sources of active species, they are, nevertheless, poorly suited for chemical applications requiring lower temperatures (in order not to completely demolish the hydrocarbon molecules to soot) and, above all, greater plasmagenic volumes to be able to act intimately on all the fluid to be treated. The plasma torch technology, for example, well established in the solid project domain, has thus been found at the same time very costly and very difficult to implement for chemical processes. However, we have obtained some improvements in the thermal plasma domain in the case of a transformation of methane with carbon dioxide or elementary oxygen in a specifically controlled electric arc, see P. JORGENSEN et al., "Process for the Production of Reactive Gases Rich in Hydrogen and in Carbon Oxide in an Electric Post-Arc, BF 2,593,493, (1986). The structure of the device placed in operation at the time unfortunately did not allow using water vapor as reagent or to work without consuming the argon necessary as plasmagenic gas of a first pilot arc. Later we used almost the same arc with higher current (20-150 A) to study the oxidation of ethylene, see K. MEGUERNES et al., "Oxidation of ethane C2H6 by CO2 or O2 in an electric arc". J. High Temp. Chem. Process, vol. 1(3), p. 71-76 (1992), without much improvement in the consumption of electric energy or of plasmagenic argon.